1. Field of the Invention
The present invention relates to a tire press in which upper and lower molds are opened and closed using a crank mechanism, and particularly to a high-precision economical tire press which is basically of a mechanically operated vertical type, but provides uniform application of a squeezing force between upper and lower molds and allows easy adjustment of the squeezing force.
2. Description of the Related Art
FIG. 4 shows a conventional tire press in which upper and lower molds are opened and closed using a crank mechanism. A lower mold 101 is mounted on a base 102, and an upper mold 103 is mounted to a top slide 104. A frame 105 is fixed to each side of the base 102. A vertical guide slot 106 is formed in each frame 105, and an arcuate roll guide 107 is formed along the top edge of each frame 105. A guide roll 108 is attached to each side of the top slide 104. As the guide rolls 108 slide along the respective vertical guide slots, the top slide 104 moves vertically. An auxiliary guide slot 120 is formed in each frame 105. An auxiliary guide roll 121 is rotatably attached to the tip of a corresponding arm 104a of the top slide 104 is guided along each auxiliary guide slot 120. Due to a restriction established by the auxiliary guide rolls 121 being guided along their respective auxiliary guide slots 120, as the guide rolls 108 roll on the arcuate roll guides 107, the top slide 104 retreats while tilting as illustrated in FIG. 4. The type of tire press with the top slide 104 moving in this manner is called the tilt-back type.
In order to obtain this tilt-back movement, a crank mechanism 110 adjacent to each frame 105 is used. In each crank mechanism 110, a crank gear 111 is rotatably supported by the base 102 while a link 114 is pivotably attached to a crank pin 112 of the crank gear 111 and to a connecting pin 113 of the top slide 104. Each crank gear 111 engages with a pinion gear 115, which is driven by an unillustrated driving unit so as to rotate the crank gear 111.
In FIG. 4, the crank pin 112 is positioned beyond its upper dead point. Thus, as illustrated, the top slide 104 is tilted at it raised position while the upper mold 103 is retreated and tilted backward from the lower mold 101. When the crank gear 111 rotates in the direction of arrow 116, each guide roller 108 attached to the top slide 104 rolls on the arcuate roll guide 107 in the direction of arrow 117. Due to the above-mentioned restriction established by the auxiliary guide roll 121 being guided along the auxiliary guide slot 120, the top slide 104 returns to a vertical posture. Subsequently, the guide roll 108 slides along the vertical guide roll slot 106. As the crank pin 112 approaches the lower dead point, the upper and lower molds 101 and 103 are closed together, and in addition the crank gear 111 produces a strong pulling force to squeeze together the upper and lower molds 101 and 103.
Because it employs a simple mechanism of the crank mechanism 110 for opening/closing molds and for squeezing molds together, the conventional tire press has an advantage that a tire can be produced at a relatively low cost. However, as described above, after the upper and lower molds 101 and 103 are closed together as a result of the crank pin 112 approaching the lower dead point, the crank gear 111 produces a strong pulling force through the so-called follow-up operation. This strong pulling force causes the top slide 104 and the base 102 to deflect and causes the link 114 to elongate, thereby producing a squeezing force. Accordingly, the squeezing force depends of the pulling force or the amount of the follow-up. The strong force of the follow-up operation, however, causes a strong reaction force to act on the crank gear 111, the tooth portion of the pinion gear 115, the plane bearing of the crank gear 111, and the driving unit including a driving motor and a speed reducer. This reaction load is about 40% larger than a load imposed on the driving unit of the crank mechanism, etc., during opening or closing the upper and lower molds 103 and 101. This implies a nonuniform driving torque with resultant poor efficiency, leaving room for improvement.
In the conventional tilt-back type tire press, since the top slide 104 retreats in a tilted position near the end of a mold opening operation, an increase in the crank radius cannot be directly converted to an increase in the vertical strode. Further, the conventional tilt-back type cannot meet requirements for a higher accuracy. This is because (1) the squeezing force is difficult to adjust and (2) the squeezing force is not applied evenly, which are problems peculiar to a vertical tire press using a crank mechanism.
The difficulty in adjusting the squeezing force, (1) above, is derived from the fact that it is actually quite difficult to adjust the amount of the follow-up, which determines the magnitude of the squeezing force. The uneven application of the squeezing force, (2) above, is derived from the fact that the top slide 104 and the base 102 deflect as shown in exaggerated form in FIG. 5 (a schematic front view of a main portion of a tire press showing a deflection thereof). This deflection causes the upper and lower molds 103 and 101 to contact each other obliquely at their outer circumferences, forming a clearance therebetween at their inner circumferences. As a result, a tire undergoing vulcanization in the upper and lower molds 103 and 101 is likely to protrude through the clearance, resulting in poor quality of a vulcanized tire product and a decreased yield. Also, unsymmetrical wear of the upper and lower molds 103 and 101 shortens their service lives. When the upper mold 103 is mounted to the top slide 104 via a mold height adjust screw, play of the mold adjust screw may absorb a deflection of the top slide 104, so that the bottom face of the upper mold 103 may substantially contact the top face of the lower mold 101 in a substantially uniform manner, thereby solving this unsymmetrical contact problem to some degree. However, since the lower mold 101 is directly mounted on the base 102, the lower mold 101 itself deforms due to the deflection of the base 1, resulting in an uneven distribution of a contact force over the contact face between the upper and lower molds 103 and 101.
In recent years, wide radial tires have become popular. A wide radial tire is vulcanized and formed in a mold called a segment mold, in which usually 6 to 8 segments are provided in an expandable/contractible manner. When this segment mold is opened/closed on the above-mentioned tilt-back type tire press, the segments are likely to be decentered when the top slide 104 tilts, resulting in poor accuracy.
In response to these circumstances, a vertical oil-hydraulic tire press has been developed in which the top slide is moved vertically by an oil cylinder. This oil-hydraulic cylinder type meets demands for uniform application and easy adjustment of a squeezing force, but still has the following disadvantages. An accompanying oil-hydraulic system makes the press structure complex, thereby increasing cost. In addition, an oil leak is unavoidable with a potential adhesion of oil to a green tire. Adhesion of oil to a green tire produces a valueless tire product.